Combined light source for projection display

ABSTRACT

High pressure mercury arc lamps are commonly used as the illumination source in many projection systems. Such lamps may be deficient in either output power or spectrum, and so it is desirable to combine the light from the lamp with light from a second light generator. The second light generator may be another mercury lamp or a solid state source, such as one or more light emitting diodes. Different ways of combining light from two light generators are described. The second light source may be an arrangement of a number of red LEDs that supplements the red light produced by the mercury light. A tunnel integrator may be used to homogenize the combined light beam and to reduce the angular separation between the light beams from the two light generators.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/745,000, filed Dec. 23, 2003, allowed which will issue as U.S. Pat.No. 7,090,357, on Aug. 15, 2006, the disclosure of which is incorporatedby reference in its entirety herein.

FIELD OF THE INVENTION

The invention relates to optical systems, and more particularly toprojector systems that use two or more light sources.

BACKGROUND

Projection systems, used for projecting an image on to a screen, useseveral different components for providing efficient illumination of theimage display unit. Projection systems typically use a lamp to generatethe illumination light, with several optical elements being disposedbetween the lamp and the image display unit to efficiently transfer thelight from the lamp to the image display unit. The image display unitmay use different mechanisms for imposing an image on the incident beamof light. For example, the image display unit may use absorption, aswith a photographic slide, polarization, as with a liquid crystaldisplay, or by the deflection of light, as with a micromechanical arrayof individually addressable, tiltable mirrors. Some image display unitsrequire that differently colored components of the image be imposed bysplitting the light beam into beams of primary colors, imposing separateimages on the primary color beams and then recombining the primary colorimages to produce the final image.

Image brightness is a key parameter for characterizing a projectionsystem. Image brightness may be affected by several factors, such as thebrightness of the lamp, the efficiency of collecting the light from thelamp, the efficiency of homogenizing the light beam, the efficiency ofrelaying the light to the image display unit, and the efficiency ofimposing the image on the light beam. It is often desirable to be ableto form the image to be as bright as possible. When the etendue of theprojection system, however, limits the amount of light in the image, thesolution to increased brightness is to use a more powerful lamp. Étendueis the product of the imager area and the solid angle determined by thef-number of the optical system. More powerful lamps are expensive andoften have a shorter lifetime. Also, more powerful lamps tend to have alarger arc, which reduces the geometrical efficiency of lightcollection.

SUMMARY OF THE INVENTION

In view of the above, there is a need to increase the brightness of thelight source used in illuminating a projection system while maintaininglong lifetime and high light collection efficiency.

In addition, high pressure mercury arc lamps are typically used forilluminating projection systems, since they have a reasonably longlifetime and a short arc length that permits high geometrical lightcollection efficiency. The high pressure mercury arc lamp, on the otherhand, produces light having a spectrum that is rich in blue and greenintensity, but relatively low in red intensity. Accordingly, to achievea desirable color temperature in the image light, a significant portionof the blue and green light is unused, which lowers the overall systemefficiency. There is a need to reduce the amount of blue and green lightemitted from the lamp that is not used in the image light beam.

In view of the above, one particular embodiment of the invention isdirected to a projector system comprising a light source and at leastone image display unit. The light source comprises a first lightgenerator producing first light. A first reflector is disposed tocollect at least part of the first light and to direct the collectedfirst light in a first direction. A second light generator producessecond light. A second reflector is disposed to collect at least part ofthe second light and to direct the collected second light in a seconddirection. A light combiner combines the first light and the secondlight to produce a combined light beam. The at least one image displayunit is illuminated by at least a part of the combined light beam fromthe light source. The at least one image display unit imposes an imageon the at least a part of the combined light beam to form an image beam.

Another embodiment of the invention is directed to a light source unitfor a projection system. The unit comprises a first light generator thatproduces first light. A first reflector is disposed to collect at leastpart of the first light and to direct the collected first light in afirst direction. A second light generator produces second light. Asecond reflector is disposed to collect at least part of the secondlight and to direct the collected second light in a second direction. Alight combiner combines the first light and the second light to producea combined light beam.

Another embodiment of the invention is directed to a light source unitfor a projection system. The light source unit includes a first lightgenerator unit producing a first light beam having a first cone angle.Light in the first light beam has a relatively broad spectrum. A secondlight generator unit produces a second light beam separated from thefirst light beam. The second light beam has a second cone anglesubstantially equal to the first cone angle. The light in the secondlight beam has a relatively narrow spectrum. A light integrator isdisposed to receive both the first and second light beams and to producea combined output beam.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates an embodiment of a projection system;

FIG. 2 schematically illustrates an embodiment of a light source inaccordance with principles of the present invention;

FIG. 3 schematically illustrates an embodiment of a light beam combiningprism pair, shown in FIG. 2, in accordance with principles of thepresent invention;

FIGS. 4A-4C schematically illustrate calculated spot diagrams of lightbeams before and after a tunnel integrator in an embodiment of aprojection system according to principles of the present invention;

FIG. 5 schematically illustrates an embodiment of a trapezoidal tunnelintegrator in accordance with principles of the present invention;

FIG. 6 schematically illustrates another embodiment of a light beamcombining prism pair in accordance with principles of the presentinvention;

FIG. 7 schematically illustrates an embodiment of a combiner forcombining light from two different light generators in a tunnelintegrator, according to principles of the present invention;

FIG. 8 presents a graph showing normalized output light spectra of ahigh pressure mercury lamp and of a red light emitting diode (LED);

FIG. 9 schematically illustrates a light source for a projection systemin which light from a lamp and from a LED source is combined using aprism pair according to an embodiment of the present invention;

FIG. 10 schematically illustrates a light source for a projection systemin which light from a lamp and from a LED source is combined usingmirror and tunnel integrator according to an embodiment of the presentinvention; and

FIG. 11 presents composite spectra of illumination light beams generatedby a high pressure mercury lamp and an LED source having differentnumbers of LEDs, in accordance with an embodiment of the presentinvention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to optical systems and isparticularly applicable to projection systems, particularly toprojection systems that use liquid crystal image display units.

A schematic illustration of a projection system 100 is presented inFIG. 1. In general terms, light 102 from a light source 104 is directedto an image display device 106. Image light 108 from the image displaydevice 106 then propagates through a projection lens system 110 forprojection on a screen. The projection system may be a rear projectionsystem, for example as is commonly found in rear projection televisions,or may be a front projection system, for example as is found in frontprojection televisions and display systems.

In the illustrated embodiment, the image display device 106 comprises acolor separation/combiner 112 that splits the light 102 into beams ofthree different primary colors 114 a, 114 b and 114 c, for example red,green and blue, each of which is directed to an associated reflectiveimage display unit 116 a, 116 b and 116 c. The color separation/combiner112 may be an x-prism, as illustrated, or may include otherconfigurations of prism, such as a Philips prism or the like. In anotherembodiment, not shown, the color separation/combiner may comprise acombination of dichroic color separators and or color prisms. Thisapproach permits the projection system 100 to form images in each of theprimary colors: combination of the primary color images in the colorseparator/combiner forms a full color image that is projected to thescreen.

In some other embodiments, the primary color images are not formedsimultaneously, but may be formed sequentially. In such systems, thereis typically no color separator/combiner. Before incidence on the imagedisplay device, however, the light passes through a color filter, sothat light of only one primary color is incident on the image displaydevice at any one time. The image display device is synchronouslycontrolled to impose an image appropriate to the color of the currentlyincident light. Images in the different primary colors are formed insequence and projected to the screen. Where the sequence of color changeis sufficiently fast, the viewer's eye integrates the images ofdifferent primary color and perceives a full color image.

Different types of image display devices may be used. One simple type ofimage display device is a photographic slide. This requires no colorseparation. Other types of image display device permit the image imposedon the light to be actively changed. Examples of such image displaydevices include liquid crystal display (LCD) units, including liquidcrystal on silicon (LCOS) units, and micromechanical displays. Oneexample of a micromechanical display is based on an array ofindividually addressable tiltable mirrors, in the DLP™ range of productssupplied by Texas Instruments, Plano Tex.

The image display device 106 may be transmissive, where the image lightis transmitted through the device. The image display device 106 mayinstead be reflective, as illustrated, where the image light isreflected from the device. The image display device 106 may even betransflective, using light that is both reflected by the device andtransmitted through the device. A polarizing beamsplitter (PBS) 118 isoften used to separate the incoming light from the reflected image lightfrom an image display device that uses polarization modulation forimposing the image on the incident light.

The light 102 from the light source 104 may be passed through one ormore homogenizer devices 120 that make the intensity profile of thelight beam more uniform. One commonly used type of homogenizer device isa tunnel integrator. A tunnel integrator is commonly a tube or rod thatis often, but is not restricted to being, rectangular in cross-section.Light passes into the integrator through the entrance aperture andundergoes several reflections at the walls before passing out throughthe exit aperture. The effect of the multiple reflections is touniformize the intensity profile of the light at the exit aperturerelative to the intensity profile at the input aperture. The light maybe reflected within the tunnel integrator via total internal reflectionor using front surface reflection. A hollow tunnel integrator does notrefract the incoming light, and so may be able to homogenize the lightover a shorter distance than an internally reflecting tunnel integrator.

The light from a light source such as a lamp is typically unpolarized.Several types of image display device, however, such as LCD-type imagedisplay devices, rely on the incident light being polarized. One way topolarize the light is simply to pass the light through a pre-polarizerand/or a PBS 118. This, however, may result in losing 50% of the light,which is inefficient. A polarization converter 122 may be used toconvert the incident light so as to increase the fraction of the lightbeam that is in the polarization state desired for incidence on theimage display device.

The projection system 100 may include several lenses for relaying lightthrough the device. For example, it is common to include a number ofrelay lenses 124 to relay an image of the output aperture of homogenizer120 onto the surfaces of the image display devices 116 a-116 c. Thisincreases the efficiency of the projection system. The image displaydevices 116 a-116 c may be connected to a controller 130, for example amicroprocessor, that controls the images imposed on the light by theimage display devices 116 a-116 c.

The projected image may be made brighter if a higher power lamp is used.Higher power lamps, however, suffer from shorter lifetime. In addition,higher power lamps typically have a larger arc, and so the geometricalcollection efficiency, the efficiency of passing the light from the arcto the image display device, may be reduced. A different approach toincreasing the brightness of the image is to combine the light from twolight sources. This permits lower power arc lamps to be used, thusproviding an increased brightness without a reduction in lifetime orgeometrical collection efficiency.

One particular embodiment of a light source 200 that combines the lightfrom two different light generators is schematically illustrated in FIG.2. In this embodiment, the light source 200 combines light from two arclamps 202 a and 202 b using a prism pair 204. The light from each arclamp 202 a and 202 b is collected and directed towards the prism pair204 by respective reflectors 206 a and 206 b. The reflectors 206 a and206 b may be elliptical reflectors. The prism pair 204 comprises twoprisms 208 and 210, separated by a gap 212 along the diagonal. The gap212 may an air space. One arc lamp 202 a is located to the side of prism210 and the central ray from lamp 202 a, shown as axis 214, forms anangle θ1 with axis 216. Another arc lamp 202 b is positioned to the sideof prism 208 and the central ray from lamp 202 b, shown as axis 218,forms an angle θ2 with axis 220.

An integrator 222, such as a tunnel integrator, also sits on axis 220.The light from the first arc lamp 202 a is substantially totallyinternally reflected within prism 210, while the light from the secondarc lamp 202 b is substantially transmitted through prisms 208 and 210.The entrance aperture 224 of the integrator 222 receives the light fromboth arc lamps 202 a and 202 b.

The operation of the prism pair 204 is now described in greater detail,with reference to FIG. 3 which shows rays of light from each of the arclamps 202 a and 202 b. Light is generally directed to the prism pair 204from the elliptical reflectors 206 a and 206 b with a conical shape. Thelight 224 a, shown in dashed lines, from lamp 202 a is incident on thesurface 226 of the prism 210. Most of the light 224 a is incident on thesurface 226 at an angle equal to, or greater than, the angle of totalinternal reflection within the prism 210, and is reflected toward theintegrator 222 as light 227. Some of the light 224 a may be incident onthe surface 226 at an angle less than the angle for total internalreflection, and is transmitted through the surface 226 and the gap 212and into the prism 208 as light 228. This light 228 is not collected bythe integrator 222.

The light 224 b, shown in solid lines, from lamp 202 b is incident onthe surface 230 of the prism 208. Most of the light 224 b is incident onthe surface 230 at an angle less than the angle of total internalreflection within the prism 208, and is transmitted through the surface230, through the prism 210 and toward the integrator 222 as light 231.Some of the light 224 b may be incident on the surface 230 at an angleequal to, or greater than, the angle for total internal reflection, isreflected at the surface 230 as light 232 and does not to propagate tothe integrator 222. This light 232 is not collected by the integrator222.

Thus, the prism pair 204 is effective at combining light collected bytwo reflectors 206 a and 206 b from two light generators 202 a and 202b, and directed to the prism pair 204, and directing the combined lightto an integrator 222. It will be appreciated that the light 227 and 231need not come to a focus, or reach its highest intensity, exactly at theentrance aperture 224 of the integrator, although the brightness may beincreased under such a condition. The reflectors 206 a and 206 b reducethe divergence of the light emitted from the arc lamps 202 a and 202 bso that a significant amount of the light enters the integrator 222.Accordingly, the light 224 a and 224 b may enter the integrator spreadout over an area. Furthermore, it will be appreciated that although theembodiment illustrated in FIG. 2 has been described using arc lamps asthe light generators, other light generators may be used, for exampleother types of lamps.

The prism pair 204 essentially permits angular combination of the lightbeams 227 and 231 having different angular light distributions in theintegrator. This is now discussed with reference to FIGS. 4A-4C, whichshow calculated spot diagram light distributions in the far field thatrepresent the angular light distribution. FIG. 4A presents a calculatedview of the spot diagram for the light from a single light generator anda single reflector. FIG. 4B presents a calculated view of the spotdiagram for light from the two light generators 202 a and 202 b aftercombination in the prism pair 204. As can be seen, the light from thetwo different light generators propagates in different directions, andthere is little overlap between the two. The calculated angularseparation between the two light beams is 23°.

A tapered tunnel integrator may be used to reduce the angular separationbetween the two light beams. An example of a tapered light tunnelintegrator 400 is schematically illustrated in FIG. 5, where theentrance aperture 502 has dimensions of x and y, and the exit aperture504 has dimensions of x and y′, where y′≢y. In the illustratedembodiment, the sides of the tunnel integrator 500 are straight, and twoof the opposing sides of the tunnel are positioned at an angle θrelative to the axis 506. The fact that the sidewalls 508 and 510 arenot parallel to the axis results in a reduction in the angular size ofthe light beam passing through the tunnel integrator 500. FIG. 4Cpresents a calculated view of the spot diagram of the light from the twolight generators 202 a and 202 b after propagation through the tunnelintegrator 500. The angular separation of the two light beams iscalculated to be 12°, significantly less than that shown in FIG. 4B. Inthe calculations that generated FIGS. 4B and 4C, it was assumed that thetunnel integrator was 40 mm long, had an entrance window 4.5 mm×4.5 mmand an exit window 4.5 mm×7.8 mm. Accordingly, the angle, θ, of thetapered sides relative to the axis 506 was ±2.36°.

The reduction in angular separation is dependent on the angle of thetunnel integrator and on the number of reflections within the tunnel.Since the optical losses associated with the tunnel integrator increasewith the number of reflections, the number of reflections and the amountby which the angular separation is reduced are left as design criteriafor specific illumination systems. Tapered tunnel integrators aredescribed in greater detail in U.S. Pat. No. 5,625,738, incorporatedherein by reference.

The tunnel integrator 500 may be a hollow integrator or may be a solidintegrator. Furthermore, the entrance aperture 502 need not be squareand the exit aperture need not be rectangular, but may take on othershapes. In many projection systems, the exit aperture is imaged to theimage display device, and so it is common for the aspect ratio of theexit aperture 504 to be the same as the aspect ratio of the imagedisplay device, otherwise the overall geometrical light collectionefficiency of the projection system may be reduced.

The prism pair 204 may be modified to reduce reflective losses for thelight transmitted through the prisms 208 and 210, as is now discussedwith reference to FIG. 6. An index matching material 612 may be placedin that portion of the air gap 212 where the light 224 b passes throughfrom prism 208 to 210. In this case, index matching means that thedifference between the refractive index of the material 612 and therefractive index of the prisms 208 and 210 is less than the differencebetween the refractive index of the prisms 208 and 210 and therefractive index of air. Consequently, reflective losses at theinterface between the prism 208 and the air gap 212 may be reduced. Ifthe refractive index of the material 612 is the same as that for theprisms, 208 and 210, then there are no reflective losses for the light224 b passing from prism 208 to prism 210. The material 612 may be anoptically transparent adhesive for holding the prisms 208 and 210together.

Unless the refractive index difference between prism 210 and thematerial 612 is sufficiently large, it is important to maintain a highrefractive index difference at the gap 212 between the prisms 208 and210 at that region where the prism 210 totally internally reflects thelight 224 a, so as to preserve high efficiency in reflecting the light224 a.

In the approach just described with reference to FIG. 2, the light fromthe two light generators is combined by providing angular separation.Another approach to combining light from different light generators isnow described with reference to FIG. 7A, in which the light from thedifferent light generators is spatially separated. Light 702, shown indashed lines, from a first light generator is incident on a turningreflector 704 that reflects the light 702 into the entrance aperture 706of an integrator 708, such as a tunnel integrator. Light 710, shown insolid lines, is directly incident on the entrance aperture 706, withoutbeing reflected by the turning reflector 704. The turning reflector 704extends only to cover a portion of the entrance aperture 706, so as topermit the light 710 to be directly incident on the aperture 706. Thelight 702 passes into substantially one part of the entrance aperturewhile the light 710 passes into substantially another part of theentrance aperture 706.

The turning reflector 704 may be any suitable type of reflectorincluding a front surface reflector, as illustrated, a rear surfacereflector, a total internal reflector or the like. The turning reflector704 may be positioned at an angle of about 45° so as to bend the light702 through an angle of about 90° towards the entrance aperture 706,although this is not a necessary condition.

If desired, another turning reflector 724 may be placed in front of theentrance aperture 706, as is schematically illustrated in FIG. 7B, so asto reflect light 726 (shown in dotted line) from another lightgenerator. In this case, light 702, light 710 and light 726 each passinto substantially different portions of the entrance aperture 706.

Thus, the approach of combining light from different sources illustratedin FIG. 7 is directed to using different parts of the entrance aperturefor different beams, while the angular light distribution of the beamsmay overlap, or may even be the same. In the approach of combining lightfrom different sources illustrated in FIGS. 2 and 3, the light beamsfrom the two sources overlap at the entrance to the integrator, but havenon-overlapping angular distributions. It will be appreciated that otherapproaches may be used for combining two or more light beams, forexample, in which light from two separate light beams is combined in away that permits partial spatial overlap of the beams at the entranceaperture to the integrator and partial overlap in the angulardistribution of the two beams.

The approaches to combining light from different light generators may beused to combine light from different types of light generatorsincluding, but not limited to, arc lamps, filament lamps, other types oflamps and also solid state light generators. These approaches may beparticularly useful for combining light from light generators where thelight is first reflectively collected and directed to an integrator. Anapproach to reflectively collecting and directing light emitted from oneor more light emitting diodes (LEDs) is described in greater detail inU.S. patent application Ser. No. 10/701,201, which is incorporatedherein by reference.

One particular use for a light source that combines light from differentlight generators is to add light having one particular spectrum to lighthaving another spectrum. Projection systems often use a high pressuremercury lamp as the light source. Some examples of these lamps includethe UHP® family of lamps available from Philips Electronics, New York,N.Y., and the VIP® family of lamps available from Osram Sylvania,Danvers, Mass. These lamps typically have a short arc size, whichpermits high geometrical collection efficiency, and long lifetime. Thespectrum of a high pressure mercury (HPM) lamp is shown as curve 802 inFIG. 8. The spectrum shows that large fractions of the output powerexist in the blue region of the spectrum, particularly between about 400nm and about 460 nm, and in the green/yellow region of the spectrum,between about 540 nm and 580 nm. The total fraction of the output powerin the red region of the spectrum, for example from about 600 nmupwards, is relatively small.

It is important to maintain a desired color balance in the imageprojected by the projection system. Maintenance of a desired colorbalance generally involves the following steps in a three-panelprojection system. First, the light from the lamp is separated intoprimary colors with color co-ordinates determined by a suitablestandard. One common standard for projected images is the RP 145-1999standard, set by the Society of Motion Picture and Television Engineers(SMPTE). This provides a desired level of color purity and can beachieved using dichroic filters in the projection system. To obtain thecorrect purity for the red and green channels, some of the yellow lightneeds to be removed. Second, after modulating the separated coloredlight beams with respective image display devices, the light isrecombined and projected. At this stage, the amount of power in theblue, green and red light beams is adjusted so as to achieve a desiredcolor temperature in the projected image. In addition to colortemperature, another useful measure of color is the color co-ordinate ofthe white point, i.e. a mixture of all three beams, red, green and blue.

An example of the color co-ordinates for a typical high pressure mercurylamp are presented in Table I, for the individual channels and fordifferent scenarios for mixing color on the screen. Some assumptionsmade in considering the data for Table I include: i) no Fresnel orpolarization losses in the projection system, ii) no vignetting orapodization of the individual channels, and iii) the green channel useslight in one polarization with the red and blue channels using the otherpolarization. This last assumption permits partial overlap of thespectra of the blue and green channels, and thus the overall efficiencyis increased. The first line in Table 1 presents idealized colorco-ordinates (0.313, 0.329) associated with the SMPTE RP 145-1999standard. The second line shows the actual values of the colorco-ordinates (0.300, 0.304) of the light emitted by a Philips UHP-100HPM lamp. Line 3 shows the different color co-ordinates for the threeseparated color bands, blue, green and red, for light generated by theUHP-100 lamp. If we assume unity weighting factors for each of the threecolor bands, as shown in line 4, then the white light on the screen hasthe color co-ordinates as shown in line 5, (0.26, 0.30). This whitelight has different co-ordinates from the white SMPTE standard ofline 1. The white light on the screen is also different in color fromwhat was emitted from the lamp, since some light is discarded in theprocess of separating the light into different color bands. Theweighting values shown in the sixth line are those values that result inwhite light on the screen having color co-ordinates as shown on line 7,similar to that of the reference standard. Thus, to produce white lightthat conforms to the SMPTE standard, only 46.1% of the green light and36.5% of the blue light is used, while all the red light is used. TABLE1 Color Co-ordinates for Uncorrected and Corrected Light from HP MercuryArc Lamp Blue Green Red White x y x y x y x y 1. SMPTE 0.155 0.07 0.310.595 0.63 0.34 0.313 0.329 2. from Hg 0.300 0.304    source 3. Colorpurity 0.147 0.081 0.326 0.59 0.638 0.346 4. Weighting 1 1 1    factor5. On the 0.26 0.30    screen 6. Weighting 0.365 0.461 1    factor 7. Onthe 0.313 0.329    screen

Accordingly, to achieve a desirable color temperature, a significantfraction of the light emitted from the high pressure mercury lamp isdiscarded. The reason for this is that the high pressure mercury lampproduces relatively little red light compared to the green and bluelight. It is desirable, therefore, to supplement the light from a highpressure mercury lamp with red light from some other source. Theaddition of red light to the output from the high pressure mercury lampeffectively increases the amount of red light in the projected image,and thus the amount of blue and green light that needs to be discardedto achieve the desired white color co-ordinate is reduced. Inconsequence, the overall system efficiency is increased.

The different approaches for combining light from different lightgenerators discussed above may be used in combining red light from oneor more LEDs with the light from a discharge lamp. An example of anormalized spectrum of light from a red LED, centered at about 650 nm,is shown in FIG. 8 as curve 804.

One embodiment of a light source 900 that uses combined light from adischarge lamp 902 a and an LED-based light source 902 b isschematically illustrated in FIG. 9. The source 900 uses a prism pair904, formed from prisms 908 and 910, with an air gap 912 therebetween tocombine the light from the discharge lamp 902 a and the LED-based lightsource 902 b, in a manner similar to that illustrated in FIG. 2.

A reflector 906 a is provided with the discharge lamp 902 a to collectthe light 916 from the lamp 902 a and direct the light 916 (shown insolid lines) towards the entrance aperture 924 of an integrator 922, forexample a tunnel integrator. The light 916 is transmitted through thetwo prisms 906 and 908. The LED-based light source 902 b employs one ormore LED units 918 that each comprise an LED emitter and a lens,typically a half-ball lens. The lens may comprise the domed encapsulantthat is commonly associated with an LED. The light emitted from the LEDemitter is reflectively collected by a respective reflector 920 that hasa reflecting surface conforming to a surface of revolution about arevolution axis. This is described in greater detail in U.S. patentapplication Ser. No. 10/701,201, incorporated herein by reference.

The LED-based light source 902 b may include more than one LED unit 918with respective reflectors 920. The reflectors 920 may each form part ofa reflector body 921. The reflector body 921 may be formed as anintegral body that contains the different reflectors 920.

The illustrated embodiment shows a light source 902 b that comprisesfour LED units 918. It will be appreciated, however, especially in viewof the teaching contained in U.S. patent application Ser. No. 10/701,201that different numbers of LED units may be used. For example, the lightsource 902 b may include six, eight or sixteen LED units. It should benoted, however, that use of these examples is not intended to limit thenumber LED units that may be used in the light source 902 b, and thatthe examples are presented for illustration purposes only.

Light 926 (shown in dashed lines) from the LED-base light source 902 bis directed by the reflector 920 to the prism pair 904. The direction ofpropagation to the prism pair 904 is such that the light 926 issubstantially totally internally reflected at the interface between theprism 908 and the air gap 912, towards the integrator entrance aperture924. Thus, the light from one or more LEDs may be efficiently collectedand combined with light from an arc lamp.

Another approach for a light source 1000, schematically illustrated inFIG. 10, is similar in some ways to the embodiment discussed withrespect to FIG. 7A, with light in one of the beams being generated in anLED-based light source. The light source 1000 comprises an arc lamp 1002a whose light 1004 is collected and directed to the entrance aperture1006 of an integrator 1008 by a reflector 1010. The reflector 1010 maybe ellipsoidal, or may be non-ellipsoidal.

An LED-based light source 1002 b, comprising one or more LED units 1012whose light is collected and directed by one or more reflectors 1014,directs light 1016 to a turning reflector 1018 that turns the light 1016into the entrance aperture 1006 of the integrator 1008. Thus, the light1004 from the lamp 1002 a and the light 1016 from the LED units 1012illuminate substantially different portions of the entrance aperture1006.

It will be appreciated that different arrangements of the light sourcesshown in FIGS. 9 and 10 may be used to combine light from LED units withlight from an arc lamp. For example, the arc lamp 902 a may bepositioned so that its light is reflected within the prism pair 904,while the LED-based light source 902 b is positioned so that its lightis transmitted through the prism pair 904. In addition, the arc lamp1002 a may be positioned so that its light is turned by the turningreflector 1018 towards the integrator 1008, while the LED-based lightsource 1002 b is positioned so that its light is transmitted into theintegrator 1008 directly.

FIG. 11 presents a comparison of the spectrum of light produced by ahigh pressure mercury arc lamp, curve 1102, with the composite spectraof light produced by a mercury lamp augmented by light produced by anumber of red LEDs. Curve 1104 is the calculated spectrum when themercury lamp is augmented by four LEDs, while curves 1106 and 1108 showspectra when the mercury lamp is augmented by eight and sixteen LEDsrespectively. As can be seen, the use of red LEDs significantly enhancesthe amount of light in the red portion of the spectrum. Consequently,less green and blue light has to be discarded in order to achieve thedesired color balance, and the use of the light emitted from the lamp ismore efficient. This is further shown in Table II, which lists the totalcollection efficiency for each assembly of LEDs and also lists thelumens efficiency for the augmented source compared to the mercury lampalone (100%). The collection efficiency for the assembly of LEDs isnormalized relative to the total light emitted by a single LED. Thus,when the mercury lamp is augmented by four LEDs, the total brightness ofwhite light complying with the SMPTE RP 145-1999 standard is 14% higherthan when the mercury lamp is used alone. TABLE II Comparison of WhiteLight Efficiency for Light Sources Augmented with Different Numbers ofLEDs Lumens efficiency for No. of LED units Unit collection efficiencybalanced white light 0 n/a 100% 4 256% 114% 8 394% 120% 16 518% 124%

The values presented in Table II were calculated using the followingassumptions. Each LED comprised a flat emitting area of 500 μm×500 μmencapsulated in a PMMA half-ball lens having a radius of 2.8 mm. EachLED was assumed to be a Lambertian emitter. The light from the LEDs wascombined into a rectangular target area 6.4 mm×12.8 mm, within acollected solid angle of ±22°. It is seen from Table II that the lumensefficiency does not increase linearly with the number of LEDs. This isdue, at least in part, to the fact that the collection area has alimited solid angle and each LED has an associated reflector of a finitesize. Consequently, as the number of LEDs increases, it becomesincreasingly more difficult to achieve the same light gains by squeezingmore LEDs into the ±22° cone subtended by the target area.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. A projector system, comprising: a light source comprising a firstlight generator producing first light; a second light generatorproducing second light; and a light combiner that combines the firstlight and the second light to produce a combined light beam; at leastone image display unit illuminated by at least a part of the combinedlight beam, the at least one image display unit modulating at least apart of the combined light beam with image information; and a lighthomogenizer unit between the light source and the at least one imagedisplay unit to homogenize the combined light beam, the first lightentering the homogenizer unit with a first angular distribution and thesecond light entering the homogenizer unit with a second angulardistribution substantially non-overlapping with the first angulardistribution.
 2. A system as recited in claim 1, further comprising aprojector lens unit that projects the image beam.
 3. A system as recitedin claim 1, further comprising at least one polarizing beamsplitter, thecombined light beam being incident on the at least one image displayunit via the polarizing beamsplitter, the at least one image displayunit being a reflective image display unit and producing a reflectedimage beam, the reflected image beam being separated from the combinedlight beam by the polarizing beamsplitter.
 4. A system as recited inclaim 1, wherein the first light generator comprises a lamp to generatethe first light.
 5. A system as recited in claim 1, wherein at least thefirst light generator comprises one or more light emitting diodes(LEDs).
 6. A system as recited in claim 1, wherein the second lightgenerator produces light at a wavelength of light that is produced bythe first light generator.
 7. A system as recited in claim 1, whereinthe first light has a spectrum different from a spectrum of the secondlight, the spectrum of the second light supplementing the spectrum ofthe first light to provide a desired color balance in the combined lightbeam.
 8. A system as recited in claim 1, wherein the first and secondlight generators are each light generators of a first type of lightgenerator comprising a lamp or a second type of light generatorcomprising a LED.
 9. A system as recited in claim 8, wherein the firstlight generator is of the same type of light generator as the secondlight generator.
 10. A system as recited in claim 1, further comprisingan image relay system to relay an image an output of the homogenizerunit to the at least one image display unit.
 11. A system as recited inclaim 1, wherein the homogenizer unit is a tunnel integrator.
 12. Asystem as recited in claim 11, wherein the tunnel integrator has anentrance aperture and an exit aperture, the entrance aperture having asize different from a size of the exit aperture.
 13. A system as recitedin claim 12, wherein the tunnel integrator is a trapezoidal tunnelintegrator.
 14. A system as recited in claim 1, wherein the first lightenters an entrance aperture of the homogenizer unit at a first portionof the entrance aperture and the second light enters the entranceaperture at a second portion of the entrance aperture different from thefirst portion.
 15. A system as recited in claim 1, wherein the at leastone image display unit comprises at least two image display units andfurther comprising a color separator disposed to separate the combinedbeam into at least two beams of different color bands to illuminate theat least two image display units with respectively light of differentcolor bands.
 16. A system as recited in claim 1 further comprising acontroller coupled to the at least one image display unit to control animage produced by the at least one image display unit.
 17. A system asrecited in claim 1, wherein the image display unit comprises at leastone of a liquid crystal display unit and a micromechanical displayhaving an array of tiltable mirrors.
 18. A system as recited in claim 1,further comprising a first reflector associated with the first lightgenerator to collect the first light and direct the collected firstlight towards the light combiner and further comprising a secondreflector associated with the second light generator to collect thesecond light and direct the collected second light towards the lightcombiner.
 19. A light source unit for a projection system, comprising: afirst light generator producing first light; a second light generatorproducing second light; a light combiner that combines the first lightand the second light to produce a combined light beam, the lightcombiner comprising a prism pair formed from first and second prisms,the first light being substantially totally internally reflected withinthe first prism and the second light being substantially transmittedthrough the first and second prisms to form a combined light beam; andan integrator, the combined light beam entering the integrator at anentrance aperture of the integrator.
 20. A system as recited in claim19, further comprising an index matching layer disposed between thefirst and second prisms, the second light that is substantiallytransmitted through the first and second prisms passing through theindex-matching layer.
 21. A system as recited in claim 20, wherein theindex-matching layer is an adhesive layer.
 22. A system as recited inclaim 21, wherein the first light totally internally reflects within thefirst prism at a surface area substantially free of the refractiveindex-matching layer.
 23. A system as recited in claim 19, wherein thefirst light generator comprises a lamp to generate the first light. 24.A system as recited in claim 19, wherein at least the first lightgenerator comprises one or more light emitting diodes (LEDs).
 25. Asystem as recited in claim 19, wherein the second light generatorproduces light at a wavelength of light that is produced by the firstlight generator.
 26. A system as recited in claim 19, wherein the firstlight has a spectrum different from a spectrum of the second light, thespectrum of the second light supplementing the spectrum of the firstlight to provide a desired color balance in the combined light beam. 27.A system as recited in claim 19, wherein the first and second lightgenerators are each light generators of a first type of light generatorcomprising a lamp or a second type of light generator comprising a LED.28. A system as recited in claim 27, wherein the first light generatoris of the same type of light generator as the second light generator.29. A system as recited in claim 19, further comprising a firstreflector associated with the first light generator to collect the firstlight and direct the collected first light towards the light combinerand further comprising a second reflector associated with the secondlight generator to collect the second light and direct the collectedsecond light towards the light combiner.
 30. A system as recited inclaim 19, wherein the integrator is a tunnel integrator.
 31. A system asrecited in claim 19, wherein the tunnel integrator is a taperedintegrator.